This invention relates to the foundry art, and, in particular to a sand binder system utilizing furfuryl alcohol monomer.
Generally speaking, the so-called furan foundry binders have been produced from partially resinified mixtures involving furfuryl alcohol, and sometimes including furfural, urea, formaldehyde, phenol and the like.
For the purposes of providing background for the present invention, the furan binders can be regarded as being categorized in one of four classes: namely, monomeric, the cooked type, polymeric, and the phenol containing type. The first type of furan binder generally involves mixing monomeric furfuryl alcohol with resorcinol, for example. The second type involves reacting furfuryl alcohol, urea, and formaldehyde, with an adjustment of pH to 4.5 or higher, by cooking the resulting mixture for 2 hours, for example, at an elevated temperature such as 100.degree. C. Under these conditions, the furfuryl alcohol polymerization is minimal but the conditions do provide for incorporation of the furfuryl alcohol monomer by condensation into a resin with the urea and formaldehyde.
The so-called polymeric type binder involves the admixture of furfuryl alcohol monomer with urea and/or urea formaldehyde with the adjustment of pH to well below 4.5, e.g., 2.0, and cooking to a desired viscosity, for example, at 100.degree. C. Ordinarily the water of condensation is stripped off under reduced pressure and the resulting polymer is quite viscous. This polymer is typically diluted with furfuryl alcohol monomer or in some cases with furfural to get a desired viscosity.
The fourth type of furan binder involves pre-condensation with phenol, and, typically, the phenol is first polymerized with formaldehyde, for example, under alkaline conditions, and the resulting phenol aldehyde is reacted or admixed with furfuryl alcohol polymer typically furfuryl alcohol-formaldehyde or furfuryl alcohol-urea-formaldehyde polymer prior to admixture with sand, for example.
We have discovered a novel binder comprising an admixture of furfuryl alcohol and hexaalkoxymethylmelamine which has tensile strengths comparable to furfuryl alcohol/formaldehyde binders, uses no formaldehyde, and thus has no formaldehyde odor. This novel binder does not have to be cooked, thus offering an advantage over the commercially available furfuryl alcohol binders which contain various amounts of formaldehyde. Furthermore, furfuryl alcohol/hexaalkoxymethylmelamine binders containing up to 20 percent water still have acceptable tensile strengths, which offers an economic advantage.
Hexamethylolmelamine is produced by the reaction of melamine with aqueous formaldehyde using an excess of formaldehyde over the theoretical ratio. The hexamethylol compound is converted to the hexamethoxy derivative by reaction with excess alcohol in the presence of an acid. The reactions occur as follows: ##STR1## In accordance with the present invention the alcohol used can include those in which R is alkyl having up to 5 carbons. The preferred hexaalkoxymethylmelamine is hexamethoxymethylmelamine, hereinafter referred to as HMMM, although hexaalkoxylmethylmelamines which include combinations of methoxy and ethoxy, combinations of methoxy and butyoxy and other hexaalkoxymethylmelamines in which the alkoxy is derived from an alcohol containing from 1 to 5 carbons, and in which the alkoxy substituents in a given molecule may be the same or different, are useful in accordance with the present invention.
Hexamethoxymethylmelamine, is a highly stable compound, available both in liquid and solid form, and is readily soluble in furfuryl alcohol. The HMMM used in all tests in this disclosure, was American Cyanamid's CYMEL 303. Because the rate of reaction of HMMM with a primary alcohol group is 20 times as fast as the self condensation reaction, we have discovered that HMMM will polymerize with furfuryl alcohol under acidic conditions to form a highly effective foundry binder.
Hexamethoxymethylmelamine is produced by the reaction of melamine with aqueous formaldehyde using an excess of formaldehyde. The hexamethylol compound is converted to the hexamethoxy derivative by reaction with excess methyl alcohol in the presence of an acid.
U.S. Pat. No. 3,346,534 issued to C. A. Bills and P. A. Yureck illustrates the use of a furfuryl alcohol/formaldehyde/urea binder and a modifying compound selected from the group comprising melamine dicyandiamide, benzoguanamine and mixtures thereof. However, the four ingredients are mixed and then cooked and used as a foundry binder for a hot core box. No indication is given as to the structure of the resulting resin. Since no methyl alcohol was added, it is believed apparent that hexamethoxymethylmelamine was not formed.
U.S. Pat. No. 2,562,898 issued to R. N. Evans and A. P. Ingrassia describes a thermal setting resin useful in the preparation of solutions for impregnating and laminating purposes, protective coatings and varnishes, or for the formation of molded articles. This thermal setting resin is produced by reacting lignin, furfuryl alcohol, melamine, and formaldehyde at an elevated temperature.
Another patent which mentions the use of melamine/formaldehyde resins and furfuryl alcohol is U.S. Pat. No. 2,518,388 issued to W. G. Simons, however, no specific examples are given using a melamine/formaldehyde resin.